The blowing of fuses and tripping of the circuit breaker seems to be getting worse and is happening with both the off-peak and booster heaters. I'm a little worried about the electrical safety and think I'll have to call a professional to check it out.

I also noted that the flashing light on the PCB is irratic. Sometimes 2 flashes a second (indicating OK I think) and sometimes slower flashes or more frequent flashes. Indicating a problem with the sensor I believe. I can understand this causing my temperature problem but the tripping and fuse blowing? Does anyone know if this could be connected? Maybe I just need to replace these sensors...

The most likely cause would be the element leaking. Other causes can be an insulation failure inside the element, thermostat problem or supply issue. Ensure whoever you have out can inspect mains electrics and has an insulation resistance tester.